The long-term goal of this project is to define the pathogenesis of calcium-containing gallstones (GS) and to develop safe, effective methods for their prevention and treatment. We believe all GS should be considered as a spectrum of cholesterol (XOL) and calcium concretions. At one end of the spectrum are so-called "pure" XOL stones (which contain visible calcium bilirubinate in their central regions). At the other end are black (calcium bilirubinate) pigment stones. In-between are "mixed" stones containing variable XOL and calcium salts, formed by reaction of Ca2+ ions with one or more of the 4 "calcium-sensitive" anions of bile: bilirubinate, carbonate, phosphate, and free fatty acids. Calcium precipitation in bile is thus a clear requisite event in the initiation and growth of all pigment stones. In addition, since all XOL stones contain calcium in their central nidus regions, we and others have postulated that calcium precipitates may serve as a nidus for nucleation of XOL. According to this view, calcium plays a crucial role in the formation of all types of GS, and further elucidation of gallstone formation requires elucidation of those factors which lead to calcium precipitation in bile. During tenure of the Consortium, great progress has been made in elucidating the thermodynamics of calcium solubility in bile. In the kinetic area, we have found that bile contains potent inhibitors of CaCO3 crystallization. Two particularly important findings were: (1) Ca2+ ions enter bile passively at the canaliculus in the convective H2O flux attending bile salt (BS) secretion, and (2) free [Ca2+] in canine hepatic and gallbladder (GB) bile is governed by Gibbs-Donnan forces induced by intraluminal, impermeant, anionic bile salts. Thus, free Ca2+ will always be present in bile and possible therapeutic reduction of [Ca2+] for prevention and treatment of GS must be accomplished by intraluminal chelation. Extensive studies have therefore been made to develop a Ca2+- chelating BS, and much progress has been made; further studies are proposed. In this proposal we address the question as to why GS, in all animal models, appear to form in the mucin gel, as opposed to the bulk phase of bile. In this regard, important observations which we have made in patients with severe obesity, who are prone to develop GS following gastric-bypass surgery, are that: (1) Both [Mucin] and free (Ca2+] are significantly increased in GB bile, and (2) Even though (Ca2+] is increased, it is actually lower than that predicted by BS-induced Gibbs- Donnan forces. To explain this, we advance a "Coulombic mousetrap" hypothesis which depends on two factors: (1) A relatively thick mucin gel, and (2) Partial protonation of the gel by H+ ions secreted by the GB epithelium (by Na+/H+ exchange).